是begin to do还是begin doing 或是都可以?

开始做某事，区别如下。
begin / start to do sth
begin / start doing sth.
1) 谈及一项长期活动或开始一种习惯时，使用doing.
How old were you when you first started playing the piano?
你几岁时开始弹钢琴？
2) begin, start用进行时时，后面动词用不定式to do
I was beginning to get angry。
我开始生起气来。
3) 在attempt, intend, begin, start 后接know, understand, realize这类动词时，常用不定式to do。
I begin to understand the truth。
我开始明白真相。
4) 物作主语时
It began to melt.

般来说,begin to do和begin doing可以互换,但在以下三种情况下,用to do：1.主语不是指人,而是it等.如：It began to rain.2.begin后接表示心理活动的词.如：begin to know believe,wonder,think等.3.begin本身是ing形...